<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <ul>
        <li>1</li>
        <li>2</li>
        <li>3</li>
        <li>4</li>
    </ul>

    <script>
        var lis = document.getElementsByTagName('li');

        for(i=0;i<lis.length; i++) {
            lis[i].onclick = function() {
                console.log(i);
            }
        }
        console.log(i);

        // 4 再往后要点击li才会打印 出来都是4  因为这时候同步执行完了，i=4，所以log（i）就是4
    </script>
</body>
</html>